RFR: (S) CODETOOLS-7902264 jcheck extension broken after upgrade to mercurial 4.3 (and python2.7)

[Tim Bell]

Hello

Mercurial 4.3 dropped support for Python 2.6. With that an API change
moved cmdutil.command to the registrar module.

     https://bugs.openjdk.java.net/browse/CODETOOLS-7902264

This fix first tries to load .command from the registrar module.  If
not found there, look next for cmdutil.command.  If that is not found,
the legacy command() implementation is used.

     http://cr.openjdk.java.net/~tbell/CODETOOLS-7902264/

Tested with hg versions 0.9.5, 3.2.3, 4.7

I will also need a sponsor since I am not a Committer in the Code Tools 
Project. [1]

Thanks in advance-

Tim

[1] http://openjdk.java.net/census#code-tools